While precision turning machines are utilized in a variety of applications, one application in which such machines are used extensively is in the manufacture of ophthalmic lenses used for the correction of various vision defects. Such lenses, which are typically manufactured from optical plastics in the United States, start with lens blanks molded with a finished front side (away from the user's eye) and a back surface (near the user's eye) which is to be shaped to the required prescription. Such shaping is typically done using either a turning, milling, or grinding process to approximate the correct lens shape. For the back surface, the amount of material removed can be substantial, a typical lens blank having a diameter of 80 mm and a thickness of 20 mm. A fining operation then removes the form error from the lens by lapping it against a master lap surface. This lapping process typically uses the equivalent of a very fine grit sandpaper. Two fining operations may be required to sufficiently eliminate the form error. The back surface is then polished to optical clarity with a felt pad and polishing compound against the master lap. Finally, the periphery of the lens is shaped to fit in the selected frame.
Since reducing both cost and response time are becoming increasingly important in the ophthalmic industry, and since capital costs and labor costs in an ophthalmic laboratory are normally fixed regardless of the number of lenses produced, a significant increase in the number of lenses which can be produced during a unit of time can result in a substantially reduced cost per lens. Further, since some optometrists having optical labs on the premises are advertising that they will provide glasses within an hour, it is necessary that the processing time for producing quality lenses be minimized. One way of accomplishing this is to reduce some of the manufacturing operations indicated above. In particular, if the part can be shaped accurately enough during the first material removal process, at least one, and preferably both, fining operations may be eliminated. The elimination of the extra fining operations not only eliminates the time and expense required for these operations, but also eliminates additional handling of the lenses, thereby further reducing both processing time and labor costs. In addition, even with only a single shaping step, it is desirable that the rate at which this step can be performed be maximized.
There is also a certain level of accuracy and surface finish which will allow even greater cost reductions in the lens production process. At present, a master hard lap is required for each prescription that a particular laboratory produces. An optical laboratory normally stores, maintains, and periodically replaces hard laps for all possible prescriptions. This may result in the storing of hundreds or even thousands of hard laps at a typical optical laboratory, resulting in a significant investment by the laboratory both in the hard laps themselves and in space for storage. Selection of the proper hard lap for a given prescription also takes time and labor, thereby adding to lab costs and reducing lab throughput. However, if the initial surface generation is precise enough, the hard laps can be eliminated and a final polishing to optical clarity can be done with a soft, conformable lap, or perhaps even with a coating.
Further, correcting astigmatism or other axial asymmetric variations in the eye requires a lens with a toric (i.e., section of a torus) or other rotationally asymmetric surface. Such lenses are more difficult to manufacture than spherical lenses because of these rotationally asymmetric surface features which require that, in a turning operation, the tool position be synchronized to the spindles angular position. Non-rotationally symmetric features such as these for precision turned parts are typically shaped by a fast tool servo. However, fast tool servos are generally used only in short travel applications involving a few hundred microns or less. Long motion is provided by a second coarse stage. Therefore, while this design can be used for components having very small asymmetric variations in depth, it is not generally an option for the production of spectacle lenses since surface feature depths can vary by 10 mm (1 cm) or more.
If an attempt is made to use the coarse stage to deal with such large variations in feature depth, then the rotational speed of the lens or other workpiece must be reduced so as to permit depth tracking by this relatively slow moving stage. However, for cutting to be achieved, a certain surface speed must be maintained between the cutting tool, for example a diamond tip, and the surface of the workpiece. If the workpiece is slowed down sufficiently to permit depth tracking by the coarse stage, then the required surface speed for cutting must be made up in other ways, generally by rotating the cutting tool. However, using a rotating cutting tool adds to the complexity and cost of the turning machine and also adds significant mass to the tool mounting assembly, further reducing the response time of this assembly. The rotation of the tool also creates a potential for vibration, the rotary motor used to rotate the tool not being a particularly stiff mount, and the rotary motor used for rotating the tool is also a source of the heat. Therefore, while the use of such rotating tool devices, with the workpiece rotating at relatively low speed, has heretofore been the preferred option for producing lenses and other components having rotationally asymmetric surface features, these techniques have been slow, costly, and have generally not been precise enough to eliminate the need for additional finishing operation.
The problems discussed above for ophthalmic lenses also apply in the manufacture of other lenses or optical components having surface features which are non-rotationally symmetric, and may also apply in the fabrication of certain non-optical, and generally opaque, components having precise, non-rotationally symmetric surface shapes, for example components formed of metals or ceramics. Such components may include, but are by no means limited to, cams and camshafts, pistons, decorative pieces, etc.
Thus, a need exists for an improved turning machine which permits precision lenses and other components which are non-rotationally symmetric to be precisely fabricated, preferably in a single shaping operation, at relatively high speed, and in particular without requiring a hard lap so as to permit the rapid, precise, low cost fabrication of such components. Such a turning machine should also be relatively simple in design, and thus easier to fabricate and of lower cost than existing machines, and also of smaller size.